Effect of latent heat of freezing on crustal generation at low spreading rates

Abstract

Lithospheric structure changes at low spreading rates (<40 mm a -1), such that steady state molten rock cannot exist above the normal Moho depth of ∼6 km. Dredge statistics of recovered lithologies indicate that the percentage of basalt is significantly decreased compared to faster ridges and that the mode of crust generation is variable. One important effect on lithospheric structure is the latent heat released when basaltic magma freezes within the mantle, instead of being transported all the way to the surface. Using thermal models, we show that freezing of melt at mantle depths buffers temperature due to the latent heat of crystallization. Two quasi-stable seafloor-spreading patterns are imposed on the model: (1) upwelling of magma along a narrow axial zone and (2) pure shear extension over a broad zone. The variability in crustal structure predicted by these models explains variability observed in dredge lithology statistics at eight slow to ultraslow spreading ridge segments. For example, the Gakkel Eastern Volcanic Zone and Sparsely Magmatic Zone are both spreading at 12 mm a -1, but the former is composed of 7% peridotite, whereas the latter is 46% peridotite. This difference can be explained by a change from a narrow axis regime with ∼2 km thick crust to a wide axis regime with a crust-mantle mix. Overall, dredge statistics and thermal models suggest that some, but not all, latent heat of ascending magmas is released at mantle depths and that various ridge axial morphologies can occur.